Sanders told the crowd he's much better suited to run this time than he was in 2016, when he lost the Democratic nomination to Hillary Clinton. Foreign policy, he said, is something he's paying much more attention to this time around.

"I was rightfully criticized the last time around because I didn't pay as much attention as I might," he said. "The economy issues, whether people have healthcare and whether they have decent paying jobs and deal with climate change is enormously important, but we have to look at the United States' role in the world as well."

Buttigieg was asked about his lack of policy experience and the fact that his campaign website doesn't have a section dedicated to his policy plans.

"I've been pretty clear where I stand on major issues," he said

"We will continue to roll out specific policy proposals," he added. "But I also think it's important we don't drown people in minutiae before we've vindicated the values that animate our policies. We go right to the policy proposals and we expect people to be able to figure out what our values must be from that."

Warren was asked about running to be the first female U.S. president.

"The way I see it is here we are in a presidential race and you stay after it every day, one might say you persist ... and that's how I'm going to be the first woman elected president of the United States," she said.

Warren, who also touted her plan to wipe out student debt, said she plans to build a strong grassroots movement and advocate for working-class Americans.

All the candidates were critical of President Donald Trump, who has become the subject of some impeachment talk since the Justice Department last week released a redacted version of the Russia investigation report from special counsel Robert Mueller. The inquiry concluded that there was no substantial evidence Trump's campaign cooperated with Russian officials or actors, but it was not able to exonerate him on the question of obstruction of justice.

Harris said she supports impeaching Trump and is confident the Democrat-controlled House could do it, but not in the GOP-controlled Senate.

"I've not seen any evidence to suggest [the Senate] will weigh on the facts instead of on partisan adherence to being protective of this president," Harris said. "So we have to be realistic about what might be the end result, but that doesn't mean the process should not take hold."

Buttigieg said Trump "deserves impeachment."

"I'm also going to leave it to the House and the Senate to figure that out," he said. "My role in the process is trying to relegate Trumpism to the dustbin of history."

Klobuchar said Trump should be held accountable and his behavior in the report was "appalling," but she didn't say whether she would support impeachment. She did make clear that she believes proposals by some Democrats, including Warren, to forgive college student loans are unrealistic.

"I wish I could staple a free college diploma under every one of your chairs. I do. Don't look. It's not there," she said. "I wish I could do that, but I have to be straight with you and tell you the truth."

She said free community college tuition and expanded Pell Grants as more realistic goals.